This invention relates to geological formation exploration in general and in particular to the utilization of resistivity measurements in the exploration of geological formations. More particularly, this invention relates to a method and apparatus for reducing anomaly induced errors in such resistivity measurements of geological formations.
It is well known in the prior art that the sedimentary portion of the earth's surface is generally comprised of successive layers or beds which generally do not have a constant thickness. Each of these beds will typically exhibit a certain resistivity characteristic which can be highly useful in the evaluation of a particular borehole with regard to the presence of hydrocarbon deposits. The resistivity characteristics of a particular formation are generally investigated by introducing a resistivity measurement sonde into the borehole. Such sondes are generally lowered into a bore hole on a cable utilizing a section of insulated cable generally known as the "bridle" which is generally disposed between the cable and the sonde.
While disposed in the borehole, the resistivity measuring sonde is utilized to generate a survey current and generally one or more focusing or "bucking" currents which are utilized to obtain deeper lateral penetration of the formation by the survey current. In previousy known resistivity measurement sondes, voltage measurements taken between electrodes disposed on the sonde are utilized to constantly adjust the amount of focusing current necessary to optimize the penetration of the survey current into the formation. A return electrode is utilized at the surface to provide a return for the various currents and permit the current measurements necessary to determine formation resistivity. A voltage reference electrode is also generally utilized and is generally located at a point between the lower part of the conductive cable and the sonde.
While this system has worked well for many years, certain field conditions have been encountered which cause errors in the calculated or apparent resistivity of the formation. One such error is induced due to an anomaly in the resistance of a formation above the sonde which causes variations in the return path of the survey and focusing currents to the return electrode. When attempting resistivity measurements in a low resistance earth formation below a highly resistive bed, the survey and focusing currents tend to return along the well casing or cable and induce a greater than normal voltage at the reference electrode disposed above the sonde. This results in an error in calculation due to the fact that the voltage differential between the sonde and the reference electrode will not be equal to the voltage differential between the sonde and the theoretical point at infinity utilized in these calculations.
This particular error, sometimes referred to as the "Groeningen" effect or "casing" effect is particularly distressing in that the resultant resistivity measurements are similar in nature to those associated with petrochemical deposits when such deposits are not present. The expense associated with drilling and testing these wellbores makes it desirable that a method and apparatus be defined to correct for or reduce the errors induced by such resistive anomalies. One method of correcting for this error involves the utilization of very low frequency currents; however, since several cycles of current are necessary to complete a single resistivity measurement, the speed at which the sonde must be operated in this method is quite slow. The expense associated with these wells and equipment dictates that these measurements be taken as rapidly as possible, thereby eliminating this approach as a viable alternative.